For a few weeks in summer, the water has retreated allowing archeologists to examine graves of prehistoric civilisations dating from the Bronze Age to the time of Genghis Khan.
Especially rich in finds are necropolises from the time an ancient Hun population that held sway at the site around some two thousand years ago.
By early July, the water will rise and cover the graves again up to 50 ft deep, frozen in winter, until it retreats briefly next summer.
Archeologists can only work here until the end of this month (June) before this mountain ‘Atlantis’ is once more submerged.
But there are fears that over several years, the damage to this “sensational” ancient site will significantly worsen, making it a “race against time” to save an array of priceless treasures, say Russian archeologists
Recent finds include the remains of two partly mummified prehistoric “fashionistas” buried with the tools of their trade.
One called ‘Sleeping Beauty’ – dressed in silk for the afterlife – was at first believed to be a priestess but is now thought to be a leather designer, the other a weaver who took her wooden spindle packed inside a sewing bag to the afterlife.
This extraordinary Atlantis site is in the mountainous Tuva Republic in southern Siberia, a favoured vacation destination for Russian president Vladimir Putin.
The sites are on the so-called Sayan Sea, a giant reservoir upstream of the Sayano-Shushenskaya Dam, Russia’s biggest power plant, on the 3,445-mile-long Yenisei River.
The reservoir covers 240 square miles but in summer the water level falls some 50ft, with 110 burials appearing on an island in the reservoir at Ala-Tey site.
Another Atlantis site called Terezin has at least 32 graves and is closer to the shore.
Other burials from the long-ago Bronze Age and the Genghis Khan, less than one thousand years ago, are also here.
“This site is a scientific sensation”, said Dr Marina Kilunovskaya from the St Petersburg Institute of Material History Culture, who leads the Tuva Archeological Expedition.
“We are incredibly lucky to have found these burials of rich Hun nomads that were not disturbed by grave robbers.”
Both the fashionista mummies “were found with fragments of leather, threads and a spindle which could have carried a special role in Huns society,” Dr Kilunovskaya told The Siberian Times.
“Huns cherished women.